1. Field of the Invention
The invention herein relates to implantable sensors for measuring bodily fluids. More particularly it relates to methods of extending the service life of such sensors.
2. Description of the Prior Art
The electrochemical oxygen sensor has been a powerful tool for revealing the role of oxygen in biological systems. Application of this sensor has been the main experimental methodology in many thousands of studies over more than 40 years. Virtually all, however, have been studies in which the sensor is used for a period of only a few days at most before recalibration is necessary. More recently there has been developed a stable oxygen sensor that is suitable for continuous application in long-term monitoring situations without the need of frequent recalibration. Such a sensor makes possible certain important oxygen monitoring applications that were not previously feasible.
This type of sensor, and its application as a component of an enzyme electrode-based system for continuously monitoring glucose, have been described in several prior patents: U.S. Pat. Nos. 4,650,547; 4,671,288; 4,703,756 and 4,781,798 the disclosures of which are incorporated herein by this reference. The system requires two oxygen sensors, one coupled to immobilized enzymes to detect oxygen modulated by the enzyme reaction, and the other to monitor the background oxygen concentration.
When a noble metal electrode (usually platinum or gold, which are slower to corrode than other metals) is immersed in an electrically conductive medium and held at a potential sufficiently cathodic with respect to an appropriate reference electrode, oxygen molecules in contact with the surface are reduced and an oxygen diffusion gradient is established, resulting in an electrical current. This phenomenon was observed and reported in the 19th century. Under controlled conditions, the reduction current may be related to oxygen concentration in the medium. This principle forms the basis of the amperometric (current-measuring) electrochemical oxygen sensor. It is recognized that the application of this principle will be influenced by factors such as: impurities in the media; pH and reaction intermediates; initial protonation of adsorbed oxygen; oxygen dissolved interstitially in the metallic electrode; the nature of the background electrolyte; and the degree of electrode surface oxide coverage.
The reaction pathways are complex. They include the kinetics of oxygen adsorption on and into the electrode and the formation of multiple metal-oxygen complexes involving short-lived intermediates. A detailed consideration of these pathways is not required here, but simplified models of the electrode reactions that are believed to explain many aspects are helpful in understanding the invention. These models are based on the observation that hydrogen peroxide is detected as an intermediate in the oxygen reduction process. One widely accepted mechanism in acidic media is the following two-step process: EQU O.sub.2 +2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 O.sub.2 (1.1) EQU H.sub.2 O.sub.2 +2H.sup.+ +2e.sup.- .fwdarw.2H.sub.2 O (1.2)
In alkaline media, a similar process has been proposed: EQU O.sub.2 +H.sub.2 O+2e.sup.- .fwdarw.HO.sub.2.sup.- +OH.sup.-(2.1) EQU HO.sub.2.sup.- +H.sub.2 O+2e.sup.- .fwdarw.3OH.sup.- (2.2)
HO.sub.2.sup.- is the ionized form of H.sub.2 O.sub.2 that is present in alkaline media. These two sets of equations indicate that oxygen reduction can proceed by either a 2 or 4 electron process on platinum in aqueous solutions. These models have proven useful in analyses of functional electrodes.
When the electrode is polarized within a certain cathodic range, the rate of electrochemical reaction is sufficiently rapid that the process becomes mass transfer limited. This results in a current "plateau," in which there is relatively little variation in current with applied potential. The electrode can be most easily operated as a sensor in this potential range.
Certain three-electrode sensors of this type, especially those described in the aforesaid U.S. patents, have been shown to have long-term stability under well-defined in vitro and in vivo conditions. It has been found, however, that after a period of operation, such sensors tend to fail in one or the other of two characteristic modes. In most cases, the current rose abruptly and returned to the original value several times over the period of a few hours before finally remaining at a high, off-scale value. In other sensors, the current suddenly began to drift downward and fell over a period of several weeks.
It would be of significant value to have a method for preventing or deferring such failures, since such would be expected to substantially increase the operating service life of the sensors. It is therefore an object of this invention to provide such a method.